1. Field
The present invention relates to an image forming apparatus capable of reducing background contamination.
2. Background Art
A background-art image forming apparatus that employs an electrophotographic method commonly applies toner as dry ink to make visible an image in an image forming operation. Such an apparatus may be a printer, a copier, a facsimile machine, and a multi-function system, for example. The multi-function system combines various image forming related functions including at least two of printing, copying, and facsimile functions.
There are mainly two types of full-color image forming apparatuses that employ an electrophotographic method. One is a single-drum-type full-color image forming apparatus, and another is a tandem-type full-color image forming apparatus.
The single-drum-type full-color image forming apparatus includes a photoreceptor and a plurality of developing devices for each of multiple colors disposed around the photoreceptor. A toner image is developed by attaching toner at each developing device to form a composite color toner image on the photoreceptor. The composite color toner image is then transferred to a transfer paper to record a color image.
By contrast, the tandem-type full-color image forming apparatus includes a plurality of photoreceptors provided in parallel, each of which has a developing device. A single color toner image is formed on each photoreceptor. The single color toner images are then transferred successively to record a composite color image.
Comparing the single-drum and tandem types, it can be seen that the single-drum type achieves a compact, cost-effective system because the single-drum includes only one photoreceptor. However, with such an arrangement it is difficult to achieve high-speed image formation because a full color image is formed by performing multiple steps repeatedly with one photoreceptor (normally four times). By contrast, tandem-type image forming apparatuses can obtain high-speed image formation, although their relatively large size can be costly. Due to increasing demand for high-speed image formation for a full color image similarly to a monochrome image, the tandem-type full-color image forming apparatus has received much attention recently.
There are two types of tandem image forming apparatuses, direct-transfer and indirect-transfer. In the direct-transfer-tandem image forming apparatus, toner images formed on each photoreceptor are transferred successively by a transfer device to a transfer paper being conveyed by a conveyance belt.
By contrast, FIG. 1 is a schematic diagram representing a configuration of the indirect-transfer-tandem-type image forming apparatus that employs an electrophotographic method and uses a two-component-development-type developing device. The indirect-transfer-tandem image forming apparatus includes photoreceptors 11, a primary transfer device 23, a secondary transfer device 24, and an intermediate transfer device 25. As shown in FIG. 1, toner images formed on each photoreceptor 11 are transferred successively to the intermediate transfer device 25 by the primary transfer device 23. The toner image on the transfer device 25 is then transferred to a transfer paper by the secondary transfer device 24.
In the indirect-transfer-tandem image forming apparatus, a secondary transfer position can be set freely. Therefore, a distance between a paper feed unit and a fixing device 28 can be made short. Accordingly, it is possible to shorten a first print time that is a time to print the first paper starting from input of a print instruction to actual printing. Further, it is possible to make fixing a paper jam easy.
The indirect-transfer-tandem-type image forming apparatus further includes a charging device to charge each photoreceptor, a cleaning device 22, and a transfer belt cleaning device 26. Thus, using the tandem image forming apparatus which employs the electrophotographic method, it is easy to obtain a high speed performance. However, there is a drawback in that the image forming apparatus becomes a large system.
FIG. 2 is a schematic diagram representing a configuration of the two-component-development-type developing device which includes a recycle unit. The two-component-development-type developing device includes a two-component-development-type developing unit 21, a toner cartridge 27 which supplies toner to the two-component-development-type developing unit 21, and a toner supply unit.
Meanwhile, to make the image forming apparatus more compact, a single-component-development-type developing unit 29 can be employed as shown in FIG. 3.
FIG. 3 is a schematic diagram representing a configuration of the indirect-transfer-tandem-type image forming apparatus which employs an electrophotographic method and uses the single-component-development-type developing device.
FIG. 4 is a schematic diagram representing a configuration of the non-magnetic single-component-development-type developing device 29 in which no recycle unit is included.
The single-component-development-type developing device 29 contains toner in the developing device 29. Therefore, it is not necessary to supply toner from the toner cartridge 27. Accordingly, the size of the image forming apparatus can be made compact by saving a space of the toner cartridge 27 and the toner supply unit, which are not necessary.
The single-component-development-type developing device 29 develops toner image with a predetermined linear velocity of a development roller 31 by forcing the development roller 31 to contact the photoreceptor 11. It may be possible to develop the toner image without contacting with the photoreceptor 11. However, an AC bias voltage may be required in addition to a development bias voltage so as to obtain sufficient development efficiency. When the development roller 31 is forced to contact the photoreceptor 11, sufficient development efficiency can be obtained.
The toner is supplied to a surface of the development roller 31 from a toner storage 33 by the supply roller 32. A toner limiting member 34 limits a toner thickness to have a predetermined thickness. However, using such single-component-development-type developing device 29, background contamination on the photoreceptor 11 will occur in a shorter time than in the two-component-development-type developing unit 21.
FIGS. 5A and 5B are illustrations of how black toner spoils other color toner images. As shown in FIG. 5A, background contamination toner which causes background contamination is reversely charged in a high percentage. Accordingly, even if the background contamination toner causes background contamination on the photoreceptor 11, the background contamination toner is not transferred at the transfer devices 23 and 24. Consequently, the background contamination toner does not cause a problem at a non-image portion of the color image on the transfer paper. However, when the black toner image forming unit is provided at a downstream side in a rotational direction of the intermediate transfer device 25 in the image forming apparatus which uses color toner, negatively charged toner on the intermediate transfer device 25 attracts the background contamination toner which is reversely charged as shown in FIG. 5B, and is transferred to the intermediate transfer device 25.
In FIGS. 5A and 5B, yellow (Y) toner image is formed on the intermediate transfer device 25. The black toner is most striking when the black toner is attached in yellow color which has a high brightness. In the reverse case in which the yellow color toner is attached in black toner, the yellow color toner is not striking. Accordingly, when the yellow toner image forming unit is provided at a downstream side in a rotational direction of the intermediate transfer device 25, there is no substantial problem. A relation between cyan color which has a high brightness and the black is similar to a relation between the yellow color and the black. Namely, when the cyan toner image forming unit is provided at a downstream side in a rotational direction of the intermediate transfer device 25, there is no substantial problem also. However, in these cases, a distance between a primary transfer position and a secondary transfer position of the color black becomes long. As a result, a first print time for monochrome print becomes longer.
Another background-art image forming apparatus is proposed to form a color image having a high degree of brilliancy and a high quality by using magnetic toner for black toner and non-magnetic toner for other color toner image. The background-art image forming apparatus further includes a second station on which a single color image such as single the color black is formed with the magnetic toner, differently of the station in which the two-component-development-type developing unit is employed.
However, the background-art image forming apparatus is a direct-transfer-tandem image forming apparatus in which toner images are transferred successively on the transfer paper conveyed by a conveyance belt. Background contamination toner may be transferred to the intermediate transfer device by being attracted by the negatively charged toner which has a reverse polarity to the background contamination toner. Therefore, it is not sufficient to avoid such background contamination problem using the background-art image forming apparatus described above.